1. Technical Field
This invention relates to the manufacture of thin strips of metal by casting and quick setting molten metal on a cold substrate which is moving at high speed, and in particular to the production of metal materials in the vitreous state by means of a rapid solidification process.
2. Description of Related Art
It is possible to impart a vitreous or amorphous structure, i.e., one which fails to exhibit any crystalline structure on X-ray exposure (see "Les verres metalliques" (Metal glasses), Praveen Chaudhari, Bill Giessen and David Turnbull, in "Pour la Science," June 1980, No. 32, p. 68, and Scientific American, Vol. 242, No. 4, at 84-96, April 1980) by a process of cooling certain molten metals or alloys at very high speed, on the order of 10.sup.6 .degree. C./second.
Such processes of producing an amorphous metallic structure generally comprise projecting a jet of molten metal onto a cooled surface which is a good heat conductor and is moving at high speed, so that the metal spreads in the form of a very thin layer over the surface.
Various processes of solidifying molten metal on cold moving surfaces have been proposed in the prior art. These processes include solidifying the metal in the following ways: inside a wheel, on a drum, on a disk, and between two rollers. The simplest and most commonly used method consists of projecting a jet of molten metal onto the outside surface of a cold metal wheel turning at high speed. The molten metal, ejected under pressure from a crucible, forms a stationary bulb on contact with the wheel which produces a rapidly solidified metal strip. The strip, under the effect of centrifugal force, separates from the cold wheel and is ejected.
Studies made of these different types of processes have revealed the influence of the boundary gaseous layer in contact with the cold surface on the quality of the edges and on the condition of the surface of the metallic strip.
These studies have led to proposals for operating under a controlled atmosphere and, notably, under low pressure, by placing all of the equipment in a closed vessel. One major disadvantage of this technique, however, resides in the volume of the vessel that must be built, particularly when the process is used on an industrial scale. Moreover, when a vacuum is applied to the system, it cannot be applied continuously because the vacuum is necessarily broken every time the strip product is removed. In addition, it has been found that, in the process of rapid solidification on a wheel, a separation of the strip takes place more rapidly when operating under vacuum than when the process is conducted in the open, and solidification is less intense.
It is, of course, possible to contemplate rapidly solidifying the metal under vacuum and continuously bringing the strip out of the vessel. However, it is difficult to adapt a vacuum vessel to a very fast-turning wheel to permit the maintenance of a satisfactory continuous vacuum while allowing the strip to emerge into the atmosphere, especially in view of the fact that separation of the metal strip from the wheel is an unstable phenomenon.
This serious disadvantage, in particular, led to the search for a technique of rapid solidification under a controlled atmosphere which did not involve centrifugal force, and with that end in view, use of the method of rapid solidification on a moving band, passing at high speed under the jet of molten metal. This method, known in principle, presents appreciable disadvantages, principal among which is vibration of the supporting band. Generally, the problem of imprecise positioning of the band results from the fast-turning pulley drive of the apparatus, the difficulty of cooling the band effectively, and the greater complexity of this method than rapid solidification on a wheel.